In the art, there are two methods available for lubricating stern tube bearings; one uses seawater for lubrication, and the other uses an oil such as a mineral oil. In the case of large-sized ship stern tube bearings where the bearing load is heavy, however, the method of lubricating with seawater, which provides only poor lubricity, is inappropriate, although it produces no marine pollution problems. In large-sized ships, oil-sealed bearing units are employed and mineral oil-based bearing oils are used. However, such mineral oil-based bearing oils are immiscible with seawater and, therefore, the lubricity markedly deteriorates upon invasion of the bearing by seawater, causing the problem of propeller shaft damaging. Furthermore, since they are poor in biodegradability, their leakage into the sea produces another problem, namely they cause marine pollution. Contrivances have been made in the aspect of the sealant and sealing system for preventing such seawater invasion and lubricant leakage into seawater; it is difficult, however, to entirely prevent oil leakage. Fats and oils of the natural origin and synthetic ester type lubricating oils, which are said to be better in biodegradability than mineral oils, have also been proposed; however, they are insoluble in seawater and, upon leakage, they form oil drops, which float on the sea surface. Such oil drops can hardly be recovered or removed, causing a problem of marine pollution. A further problem is that when they are mixed with invading seawater, they are readily hydrolyzed, hence the lubricity readily decreases (refer to Tribologist, vol. 48, p. 114-121 (2003); Japanese Kokai Publication Hei-05-331481; Japanese Kokai Publication Hei-11-323373).